Comorbid anxiety disorders affect as many as 80% of youth with autism spectrum disorders (ASD), causing substantial distress and impairment over and above the autism spectrum diagnosis alone. Cognitive behavioral therapy (CBT) is the gold standard treatment among typically developing youth with an anxiety disorder but has yet to be adapted to meet the unique needs of adolescents with ASD and comorbid anxiety. This project will develop a CBT protocol for anxiety and comorbid ASD in early adolescence, as well as a credible comparison protocol (Relaxation Training; RT) for use in a future multisite clinical trial to test the efficacy of CBT in this population. With more youth being diagnosed with ASD after the early childhood period, a treatment protocol such as that proposed could contribute to public health efforts to address the mental health needs of early adolescents with ASD.